“In the history of military technology, new ways of destroying adversaries have presented themselves at irregular, though ever shorter, intervals. Yet technological innovation has a bleak dialectic: advances in warfare usually require adaptive mechanisms purchased at tactical cost. American soldiers of 2006, for example, waddling like armadillos in their unwieldy carapaces of body armor — which improve the odds of surviving wounds otherwise lethal — make excellent targets for ragged insurgents using the weapons of an earlier generation.”